Unit 2 - Cells and Cellular Transport

prokaryote

An informal term for an organism in either domain Bacteria or domain Archaea.

protocell

An abiotic precursor of a living cell that had a membrane-like structure and that maintained an internal chemistry different from that of its surroundings.

ribozyme

An RNA molecule that functions as an enzyme, such as an intron that catalyzes its own removal during RNA splicing.

abiotic synthesis of organic molecules

a process, by which, organic molecules are produced without living things

Reducing atmosphere

an atmosphere that adds electrons to molecules. It is hypothesized that early Earth's atmosphere may have existed in this state which promotes the formation of organic compounds

polymerization

the joining of small molecules to form macromolecules.

formation of self-replicating molecules

a hypothesized necessary step in the formation of life which makes inheritance possible

endosymbiosis

A mutually beneficial relationship between two species in which one organism lives inside the cell or cells of another organism.

endosymbiont theory

The theory that mitochondria and plastids, including chloroplasts, originated as prokaryotic cells engulfed by host cells. The engulfed cell and its host cell then evolved into a single organism.

selective permeability

A property of biological membranes that allows them to regulate the passage of substances across them.

amphipathic

Having both a hydrophilic region and a hydrophobic region.

fluid mosaic model

The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.

integral protein

A transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both sides of the membrane (or lining the channel in the case of a channel protein).

peripheral protein

A protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer.

glycolipid

A lipid with one or more covalently attached carbohydrates.

glycoprotein

A protein with one or more covalently attached carbohydrates.

transport protein

A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.

channel protein

A type of transport protein that functions by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through a membrane.

aquaporin

A channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane.

carrier protein

A type of transport protein that holds onto its passengers and changes shape in a way that shuttles them across a membrane.

diffusion

The random thermal motion of particles of liquids, gases, or solids. In the presence of a concentration or electrochemical gradient, diffusion results in the net movement of a substance from a region where it is more concentrated to a region where it is less concentrated.

concentration gradient

A region along which the density of a chemical substance increases or decreases.

passive transport

The diffusion of a substance across a biological membrane with no expenditure of energy.

osmosis

The diffusion of free water molecules across a selectively permeable membrane.

tonicity

The ability of a solution surrounding a cell to cause that cell to gain or lose water.

isotonic

Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell.

hypertonic

Referring to a solution that, when surrounding a cell, will cause the cell to lose water.

hypotonic

Referring to a solution that, when surrounding a cell, will cause the cell to take up water.

osmoregulation

Regulation of solute concentration and water balance by a cell or organism.

turgor pressure

A back pressure exerted by a cell wall that occurs when the cell swells.

turgid

Swollen or distended, as in plant cells. Occurs in a walled cell if it has a lower water potential than its surroundings, resulting in entry of water.

flaccid

Limp. Lacking turgor (stiffness or firmness), as in a plant cell in surroundings where there is a tendency for water to leave the cell. Occurs in a walled cell if it has a higher water potential than its surroundings, resulting in the loss of water.

plasmolysis

A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall; occurs when the cell loses water to a hypertonic environment.

facilitated diffusion

The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure.

ion channel

A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.

gated channel

A transmembrane protein channel that opens or closes in response to a particular stimulus.

active transport

The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy.

sodium-potassium pump

A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.

membrane potential

The difference in electrical charge (voltage) across a cell's plasma membrane due to the differential distribution of ions. Affects the activity of excitable cells and the transmembrane movement of all charged substances.

electrochemical gradient

The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).

proton pump

An active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process.

cotransport

The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

exocytosis

The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.

endocytosis

Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane.

phagocytosis

A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

receptor-mediated endocytosis

The movement of specific molecules into a cell by the inward budding of vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.

organelle

Any of several kinds of membrane-enclosed structures with specialized functions, suspended in the cytosol of eukaryotic cells.

cytosol

The semifluid portion of the cytoplasm.

eukaryotic cell

A type of cell with a membrane-enclosed nucleus and membrane-enclosed organelles. Members include protists, plants, fungi, and animals.

prokaryotic cell

A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles. Members include bacteria and archaea.

nucleoid

A non-membrane-enclosed region in a prokaryotic cell where its chromosome is located.

cytoplasm

The contents of the cell enclosed by the plasma membrane; in eukaryotes, the portion exclusive of the nucleus.

plasma membrane

The membrane at the boundary of every cell that acts as a selective barrier, regulating the cell's chemical composition.

chromosome

A cellular structure consisting of one DNA molecule and associated protein molecules.

ribosome

A complex of rRNA and protein molecules that functions as a site of protein synthesis in the cytoplasm; consists of a large subunit and a small subunit. In eukaryotic cells, each subunit is assembled in the nucleolus.

cell wall

A protective layer external to the plasma membrane in the cells of plants, prokaryotes, fungi, and some protists. Polysaccharides such as cellulose (in plants and some protists), chitin (in fungi), and peptidoglycan (in bacteria) are important structural components.

flagellum

A long cellular appendage specialized for locomotion.

nucleus

The organelle of a eukaryotic cell that contains the genetic material in the form of chromosomes, made up of chromatin.

nuclear envelope

In a eukaryotic cell, the double membrane that surrounds the nucleus, perforated with pores that regulate traffic with the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum.

chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, it exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope.

nucleolus

A specialized structure in the nucleus consisting of chromosomal regions containing ribosomal RNA (rRNA) genes along with ribosomal proteins imported from the cytoplasm; site of rRNA synthesis and ribosomal subunit assembly.

endomembrane system

The collection of membranes inside and surrounding a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles; includes the plasma membrane, the nuclear envelope, the smooth and rough endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, and vacuoles.

vesicle

A membranous sac in the cytoplasm of a eukaryotic cell.

endoplasmic reticulum

An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.

smooth ER

That portion of the endoplasmic reticulum that is free of ribosomes.

rough ER

That portion of the endoplasmic reticulum with ribosomes attached.

glycoprotein

A protein with one or more covalently attached carbohydrates.

transport vesicle

A small membranous sac in a eukaryotic cell's cytoplasm carrying molecules produced by the cell.

golgi apparatus

An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum and synthesize some products, notably noncellulose carbohydrates.

lysosome

A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.

phagocytosis

A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

vacuole

A membrane-bounded vesicle whose specialized function varies in different kinds of cells.

food vacuole

A membranous sac formed by phagocytosis of microorganisms or particles to be used as food by the cell.

contractile vacuole

A membranous sac that helps move excess water out of certain freshwater protists.

central vacuole

In a mature plant cell, a large membranous sac with diverse roles in growth, storage, and sequestration of toxic substances.

mitochondrion

An organelle in eukaryotic cells that serves as the site of cellular respiration; uses oxygen to break down organic molecules and synthesize ATP.

chloroplast

An organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.

endosymbiont theory

The theory that mitochondria and plastids, including chloroplasts, originated as prokaryotic cells engulfed by host cells. The engulfed cell and its host cell then evolved into a single organism

plastid

One of a family of closely related organelles that includes chloroplasts, chromoplasts, and amyloplasts. Found in the cells of photosynthetic eukaryotes.

peroxisome

An organelle containing enzymes that transfer hydrogen atoms from various substrates to oxygen (O2), producing and then degrading hydrogen peroxide (H2O2).

cytoskeleton

A network of microtubules, microfilaments, and intermediate filaments that extends throughout the cytoplasm and serves a variety of mechanical, transport, and signaling functions.

microtubule

A hollow rod composed of tubulin proteins that makes up part of the cytoskeleton in all eukaryotic cells and is found in cilia and flagella.

centrosome

A structure present in the cytoplasm of animal cells that functions as a microtubule-organizing center and is important during cell division. Has two centrioles.

centriole

A pair of structures in the centrosome of an animal cell, each composed of a cylinder of microtubule triplets arranged in a 9+0 pattern.

cilium

A short appendage containing microtubules in eukaryotic cells. Some are specialized for locomotion or moving fluid past the cell and have a "9 + 2" arrangement.

microfilament

A cable composed of actin proteins in the cytoplasm of almost every eukaryotic cell, making up part of the cytoskeleton and acting alone or with myosin to cause cell contraction; also known as actin filament.

myosin

A type of motor protein that associates into filaments that interact with actin filaments, causing cell contraction.

intermediate filament

A component of the cytoskeleton that includes filaments intermediate in size between microtubules and microfilaments.

extracellular matrix

The meshwork surrounding animal cells, consisting of glycoproteins, polysaccharides, and proteoglycans synthesized and secreted by the cells.

collagen

A glycoprotein in the extracellular matrix of animal cells that forms strong fibers, found extensively in connective tissue and bone; the most abundant protein in the animal kingdom.